Understanding the Effects of Climate and Water Management on Carbon and Energy Fluxes for Restored Wetlands in the Sacramento - San Joaquin Delta, California, USA

Our research efforts focus on the differences in carbon and energy fluxes due to the effects of water management on two 3.5-hectare restored wetlands on Twitchell Island in the Sacramento-San Joaquin Delta (Delta). These flux measurements are part of an ongoing, long-term study investigating management techniques to mitigate subsidence through atmospheric carbon sequestration and soil carbon storage. Wetlands were established in 1997, with the western wetland managed at a water depth of 25cm and the eastern wetland managed at a depth of 55cm. Over the past 14 years, the western pond has developed into a dense canopy of emergent marsh species with some floating vegetation. The eastern wetland is a combination of the same emergent marsh species and floating vegetation as the western wetland, but it also includes areas of open water, submerged vegetation, and algae. Carbon and energy flux measurements are collected using the eddy covariance method, comprised of a CSAT3 sonic anemometer, an open-path CO2/H2O infrared gas analyzer, and a closed-path tunable diode laser fast methane sensor. The Delta is a unique place as the temperate climate and clear summer skies are conducive for maximum daily CO2 uptake rates to be on the order of 30 μmol m-2 s-1 or higher. These elevated rates of CO2 uptake were measured in the eastern wetland during 2002 through 2004. However, in 2010, maximum CO2 uptake rates were only about 10 μmol m-2 s-1. We hypothesize that large mats of accumulating senescent material have slowed or stopped the growth of the emergent marsh species, which were not present during the measurements taken in 2002 through 2004. Additionally, we added CH4 flux measurements in 2010, and the anaerobic conditions created by permanent flooding resulted in rates of 250 nmol m-2 s-1 or higher. CH4 values are some of the highest observed compared to other Delta flux studies (rice, pasture, and natural wetlands), which yield measurements ranging from 10 - 100 nmol m-2 s-1. We tested our hypothesis in 2011 by moving the eddy covariance tower to the western pond, where the emergent marsh species are denser. Here we present results showing diurnal and seasonal trends of CO2 fluxes for years 2002-2004 and 2010-2011, and CH4 fluxes for years 2010-2011. To understand the influence of seasonal variability, we normalized fluxes with abiotic and biotic conditions, such as air, leaf, and water temperatures, differences in humidity, and changes in daily and seasonal variations in solar radiation. We also present results from a footprint algorithm designed to examine seasonal variances in the footprint from the 2010-2011 flux measurements. Lastly, we show and compare results from other ongoing flux studies in the Delta.